1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may include an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It will be appreciated that, whether or not a lithographic apparatus operates in stepping or scanning mode, it is vital that the patterned beam is directed onto the appropriate target portion of the substrate surface. In many circumstances multi-layer structures are built up on the surface of the substrate as a result of a sequence of lithographic processing steps and it is of course vital that the successive layers formed in the substrate are correctly in register with each other. Thus, great care is taken to ensure that the position of the substrate relative to the beam projection system is accurately known. Generally, this is achieved by positioning the substrate in a known orientation on a substrate table, for example by engaging an edge of the substrate with a support surface on the substrate table and then using a reference mark on the substrate to identify exactly where the substrate is relative to the substrate table. A metrology system is then used to control relative displacement between the substrate and the beam projection system. The reference mark establishes a datum position relative to which all displacements are measured.
As the size of substrates has increased it has become more difficult to rely upon a single reference mark on a substrate to determine exactly where the substrate is relative to a datum position. To address this problem it has been known to put multiple reference marks on a single substrate, so that the metrology system which monitors substrate position can be recalibrated after a relatively small displacement as between the projection system and the substrate. However, in some applications this is not possible, for example in the manufacture of large flat panel displays (FPD's) relying upon large numbers of liquid crystal devices (LCD's) as a pattern generator system. In the case of FPD's, very large thin glass substrates are contemplated (e.g., about 1.85×2.2 meters) with a thickness of less than about 1 mm. On each glass substrate, several panels (e.g., 4, 6 or 9) may be defined, each panel corresponding to a single product (e.g., a computer monitor screen, a television screen, or the like) with sizes ranging currently from about 10 inches to about 55 inches (measured diagonally from corner to corner). Within each panel area on a single substrate, individual LCD's each defining individual pixels are arrayed. To ensure high optical transmission efficiency and to avoid irregular visually apparent optical effects, no alignment features can be formed within any one panel. Thus, alignment features must be arranged around the periphery of the glass substrate and between adjacent panels. As the size of individual panels increases, the distance between adjacent alignment features also increases. Thus, less alignment information can be acquired from alignment features formed on the surface of the substrate.
One source of positional errors as between the projection system and the substrate is thermal expansion. If two marks are made on the surface of the substrate, and the temperature of the substrate is then increased, the spacing between those marks will increase as the result of thermal expansion. Therefore, a structure formed on the surface of a substrate when the substrate is at one temperature will not register with a subsequently formed structure if the subsequently formed structure is the result of a later exposure when the temperature of the substrate has changed. This is a well known problem that has been addressed in the past by taking great care to maintain substrates at a predetermined datum temperature. However, in practice this is difficult to achieve, particularly with large substrates. It is now possible to contemplate substrates of very large size, for example substrates having outside dimensions of the order of two meters. Very small temperature variations across substrates of such size can result in expansions and contractions which are significant in the context of for example LCD display panels.
Therefore, what is needed is a system and method that allow for accurate alignment between exposures, while substantially eliminating or reducing misalignment caused by thermal expansion.